This invention relates to processes for the preparation of certain nucleoside analogues, to new intermediate compounds useful in such processes, and to the preparation of intermediate compounds useful in such processes.
Compounds which are potent inhibitors of purine nucleoside phosphorylase and are useful for suppressing T-cell function and/or treating and/or preventing infections caused by protozoan parasites are described in Biochemistry, 1998, 37, 8615-8621 and in our co-pending PCT International Patent Application No. PCT/US98/21717 (WO 99/19338).
There remains a need for alternate, simpler methods of preparing these compounds.
It is therefore an object of the present invention to provide an alternate process for preparing these compounds, which will at least provide the public with a useful choice.
In a first aspect, the present invention provides a process of preparing a compound of the formula 
wherein B is chosen from OH, NH2, NHR, H or halogen; D is chosen from OH, NH2, NHR, H, halogen or SCH3; R is an optionally substituted alkyl, aralkyl or aryl group; and Z is selected from OH, hydrogen, halogen, hydroxy, SQ or OQ, Q is an optionally substituted alkyl, aralkyl or aryl group; or a tautomer thereof; or a pharmaceutically acceptable salt thereof; or an ester thereof; or a prodrug thereof, wherein the process comprises the following steps:
(a) reacting a compound of the formula (II) 
xe2x80x83wherein Zxe2x80x2 is a hydrogen or halogen atom, a group of formula SQ or OQ, or a trialkylsilyloxy, alkyldiarylsilyloxy or optionally substituted triarylmethoxy group and Q is an optionally substituted alkyl, aralkyl or aryl group, sequentially with a halogenating agent and a sterically hindered base to form an imine;
(b) condensing the imine thus prepared with an anion produced by abstraction of the bromine or iodine atom from a compound of formula (XIX): 
xe2x80x83wherein R5 is a bromine or iodine atom, R6 is an N-protecting group, Bxe2x80x2 and Dxe2x80x2 are independently selected from H, OR7 and N(R8)2, and R7 and R8 are O- and N-protecting groups respectively, to produce a 1-C-(pyrrolo[3,2-d]pyrimidin-7-yl)-1,4-dideoxy-1,4-imino-2,3-O-isopropylidene-D-ribitol derivative of formula (SAC): 
xe2x80x83wherein R9 is a hydrogen atom, Zxe2x80x2 is as defined above for compounds of formula (II) and R6, Bxe2x80x2 and Dxe2x80x2 are as defined above for compounds of formula (XIX);
(c) optionally, converting the compound of formula (XX) to a compound of formula (XX) wherein Zxe2x80x2, R6, Bxe2x80x2 and Dxe2x80x2 are as defined above but R9 is alkoxycarbonyl or aralkoxycarbonyl, or optionally, where Zxe2x80x2 in the compound of formula (XX) is trialkylsilyloxy, alkyldiarylsilyloxy or optionally substituted triarylmethoxy, converting the compound of formula (XX) to a compound of formula (XX) wherein R6, R9, Bxe2x80x2 and Dxe2x80x2 are as defined above but Zxe2x80x2 is OH; and
(d) N- and O-deprotecting the compound of formula (XX) prepared from step (b) or (c), by acid- or alkali-catalyzed hydrolysis or alcoholysis or catalytic hydrogenolysis as required for the O- and N-protecting groups in use, to produce a compound of the formula (I) as defined above.
Where a pharmaceutically acceptable salt, ester or prodrug of the compound of formula (I) is desired, the process will also include the further step of converting the compound of formula (I) thus prepared to the desired pharmaceutically acceptable salt, ester or prodrug, using methods known in the art.
In a preferred embodiment, the halogenating agent used in step (a) is N-chlorosuccinimide.
In a preferred embodiment, the hindered base used in step (a) is lithium tetramethyl piperidide.
Preferably, in step (b) the bromine or iodine atom is abstracted from the compound of formula (XIX) using butyllithium or magnesium.
Preferably, the N-protecting group R6 in the compound of formula (XIX) is an alkoxymethyl group (such as benzyloxymethyl), a silyl group (such as tert-butyldimethylsilyl) or an arylmethyl group (such as benzyl).
Preferably, the O-protecting group R7 is an alkyl or arylmethyl group (such as methyl, tert-butyl or benzyl).
Preferably, each N-protecting group R8 is independently an arylmethyl group (such as benzyl or 4-methoxybenzyl), or the two R8 groups together form the 2,4-hexadien-2,5-yl group.
In a further aspect, the present invention provides a compound of formula (XX): 
wherein R9 is a hydrogen atom, an alkoxycarbonyl or aralkyloxycarbonyl group, Zxe2x80x2 is a hydrogen or halogen atom, a hydroxy group, a group of formula SQ or OQ, or a trialkylsilyloxy, alkyldiarylsilyloxy or optionally substituted triarylmethoxy group and Q is an optionally substituted alkyl, aralkyl or aryl group, and R6 is an N-protecting group, Bxe2x80x2 and Dxe2x80x2 are independently selected from H, OR7 and N(R8)2, and R7 and R8 are O- and N-protecting groups respectively.
Preferred are compounds of formula (XX) wherein R9 is a hydrogen atom or a tert-butoxycarbonyl group, Zxe2x80x2 is a hydroxy group, a tert-butyldimethylsilyloxy or methylthio group, and R6 is a benzyloxymethyl, allyl, tert-butyldimethylsilyl, 2-(trimethylsilylethoxy)methyl or benzyl group, Bxe2x80x2 is a methoxy, tert-butoxy or benzyloxy group and Dxe2x80x2 is a hydrogen or fluorine atom, a dibenzylamino group or a bis(4-methoxybenzyl)amino group.
In a further aspect, the present invention provides a process of preparing a compound of formula (XX) as defined above, wherein the process comprises the steps (a) and (b) as defined above for preparing compounds of the formula (I); and optionally
(a) converting R9 from a hydrogen atom to an alkoxycarbonyl or aralkyloxycarbonyl group by reaction with an alkoxycarbonylating or aralyloxycarbonylating reagent; or
(b) converting Zxe2x80x2 from a trialkylsilyloxy, alkyldiarylsilyloxy or optionally substituted triarylmethoxy group to a hydroxy group by reaction with a source of fluoride or acid.
In a further aspect, the present invention provides a process of preparing a compound of the formula (I) as defined above, or a pharmaceutically acceptable salt, ester or prodrug thereof, comprising the step of N- and O-deprotecting a compound of the formula (XX) as defined above by acid- or alkali-catalyzed hydrolysis or alcoholysis or catalytic hydrogenolysis, as required for the O- and N-protecting groups in use, to produce a compound of the formula (I) as defined above.
In further aspects, the present invention provides compounds of the formula (I) and (XX) when prepared by processes as defined above.
In a further aspect, the present invention provides a process of preparing the compound 2-amino-3H,5H-pyrrolo[3,2-d]pyrimidin-4-one, comprising the step of reacting the compound 2-(N-dimethylaminomethylene)amino-6-(2-dimethylaminovinyl)-5-nitropyrimid-4-one with a reagent capable of reducing the nitro group, to produce the compound 2-amino-3H,5H-pyrrolo[3,2-d]pyrimidin-4-one.
Preferably, the reducing agent used is aqueous sodium dithionite. Alternatively, the reduction may be carried out using catalytic hydrogenation. Where the reagent is aqueous sodium dithionite (a preferred reagent), the process is preferably carried out at elevated temperature, preferably at about 100xc2x0 C.
Preferably, the process includes the initial step of preparing the compound 2-(N-dimethylaminomethylene)amino-6-(2-dimethylaminovinyl)-nitropyrimidin-4-one by reacting the compound 2-amino-6-methyl-5-nitro-pyrimidin-4-one with a reagent capable of effecting dialklaminomethylation.
Preferably, the reagent used to effect the dialkylaminomethylation is a combination of DMF dimethylacetal and DMF.
Preferably, this step is carried out with heating at about 100xc2x0 C.
Alternatively, the reagent may be Bredereck""s reagent (t-butoxy bis(dimethylamino)methane).
In a further aspect, the invention provides the compound 2-amino-3H,5H-pyrrolo[3,2-d]pyrimidin-4-one when prepared by a process as defined above.
In a further aspect, the invention provides a compound of the formula: 
wherein Y is an unsubstituted or substituted alkyl or arylalkyl group having 1 to 8 carbon atoms.
In another aspect, the invention provides a process of preparing the compound 3H,5H-pyrrolo[3,2-d]pyrimidin-4-one, comprising the step of reacting a compound of the formula (A) defined above with a reagent capable of delivering a formyl equivalent.
Preferably, the reagent used is formamidine acetate.
Preferably, the compound of formula (A) is the compound 3-amino-2-ethoxycarbonylpyrrole.
Conveniently, the reaction is carried out in refluxing ethanol.
In still a further aspect, the invention provides a process of preparing a compound of the formula (A) defined above, comprising the following steps:
(a) reacting isoxazole with alkoxide ions in the presence of an alcohol;
(b) quenching the reaction with an acid;
(c) reacting the resulting reaction mixture with a dialkyl aminomalonate;
(d) isolating an organic phase of the resulting reaction mixture and reducing it to a residue; and
(e) reacting the residue from the organic phase with a base in a protic or aprotic solvent to produce a compound of formula (A).
Preferably, step (e) comprises reacting the residue from the organic phase with alkoxide ions in excess alcohol.
In a preferred embodiment, the compound of formula A is 3-amino-2-ethoxycarbonylpyrrole. In this embodiment, the alcohol used in excess in step (e) is ethanol and the alkoxide ions are ethoxide ions.
In still a further aspect, the present invention provides a process of preparing the compound 3H,5H-pyrrolo[3,2-d]pyrimidin-4-one, comprising the following steps:
(a) preparing a compound of formula (A) by a process as defined above; and
(b) reacting the compound of formula (A) thus prepared with a reagent capable of delivering a formyl equivalent.
In another aspect, the invention provides the compound 3H,5H-pyrrolo[3,2-d]pyrimidin-4-one when prepared by a process as defined above.
In yet a further aspect, the present invention provides a process for preparing the compound of formula 3 or 4 (defined below), respectively: 
comprising the steps of:
(1) carrying out N,O-protection of the compound 3H,5H-pyrrolo[3,2-d]pyrimidin-4-one or 2-amino-3H,5H-pyrrolo[3,2-d]pyrimidin-4-one, respectively;
(2) brominating the protected compound at C7;
(3) lithiation of the resulting brominated compound;
(4) addition of the resulting lithiated species to a protected imine, specifically a 2,3,5-tri-O-protected 1,N-dehydro-1,4-dideoxy-1,4-imino-D-ribitol; and
(5) carrying out N,O-deprotection of the resulting species to produce the compound 3 or 4, respectively, as defined above.
In still a further aspect, the present invention provides a method of preparing the compound 3 or 4, respectively, as defined above, comprising the following steps:
(a) preparing the compound 3H,5H-pyrrolo[3,2-d]pyrimidin-4-one or 4-amino-3H,5H-pyrrolo[3,2-d]pyrimidin-4-one, respectively, by a method as defined above;
(b) carrying out N,O-protection of the compound 3H,5H-pyrrolo[3,2-d]pyrimidin-4-one or 2-amino-3 H,5H-pyrrolo[3,2-d]pyrimidin-4-one, respectively;
(c) brominating the protected compound at C7;
(d) lithiation of the resulting brominated compound;
(e) addition of the resulting lithiated species to a protected imine, specifically a 2,3,5-tri-O-protected 1,N-dehydro-1,4-dideoxy-1,4-imino-D-ribitol; and
(f) carrying out N,O-deprotection of the resulting species to produce the compound 3 or 4, respectively, as defined above.
Although the present invention is broadly as defined above, it is not limited thereto and also includes embodiments of which the following description provides examples.
As defined above, the present invention provides a new process of preparing the compounds of formula (I) as defined above, or tautomers, pharmaceutically acceptable salts, esters or prodrugs thereof. These compounds and various methods for preparing them are described in International Patent Publication No. WO 99/19338, the contents of which are incorporated by reference.
The process of the present invention represents an alternate method of preparing the compounds of formula (I). The process of the present invention is a convergent synthesis route rather than a linear one. This means that it has the advantage of being able to provide higher yields of the compounds of formula (I) as well as providing a more practical route to these compounds than those previously described.
The process of the present invention comprises the step of reacting a compound of the formula (II) (as defined above) sequentially with a halogenating agent, such as N-chlorosuccinimide, and a sterically hindered base, to form an imine. The imine thus prepared is then condensed with an anion produced by abstraction of the bromine or iodine atom from a compound of the formula (XIX) as defined above, to form a compound of formula (XX) as defined above. This is followed by N- and O-deprotection of a compound of formula (XX) to produce a compound of formula (I). If desired, the compound of formula (I) thus prepared may be converted into a pharmaceutically acceptable salt, ester or prodrug thereof, using methods known in the art.
Particularly suitable as N-protecting groups R6 in the compound of formula (XIX) are alkoxymethyl groups such as benzyloxymethyl, silyl groups such as tert-butyldimethylsilyl, and arylmethyl groups such as benzyl.
Particularly suitable as O-protecting groups R7 in the compound of formula (XIX) are alkyl or arylmethyl groups such as methyl, tert-butyl or benzyl.
Particularly suitable as N-protecting groups R8 in the compound of formula (XIX) are arylmethyl groups such as benzyl or 4-methoxybenzyl, or the two R8 groups may together form the 2,4-hexadien-2,5-yl group.
Examples of preferred values for the groups R and Q in the compounds of formulae (I) and (II) are methyl, ethyl and benzyl. Suitable substituents for the groups R and Q include halogen, preferably fluorine.
The compounds of formula (XIX) defined above may be prepared by conventional methods such as those detailed herein.
In particular, unprotected deazapurines can be converted by conventional methods into their protected forms (XIX).
Thus, 9-deazahypoxanthine can be treated with a chlorinating reagent, preferably phosphoryl chloride, to form the 6-chloro-9-deazapurine. After N-protection, the chlorine is displaced with alkoxide ion. The resulting N,O-protected deazapurine is then 9-halogenated.
Alternatively, known 5-nitro-6-methylpyrimidine derivatives can first be converted into suitably protected intermediates, and then cyclized to the corresponding deazapurines, for example by reaction with tert-butoxy-bis(dimethylamino)methane, and then N-protected.
Thus, 5-nitro-6-methyl-pyrimidin-2,4-dione can be sequentially (i) chlorinated, preferably with a reagent such as phosphoryl chloride; (ii) reacted with alkoxide to displace chloride; (iii) treated with a reagent capable of delivering a formyl equivalent, especially a dimethylaminomethylating agent, preferably Bredereck""s reagent; (iv) treated with a reducing agent to reduce the nitro group and cause cyclization; and in either order (v) N-protected and (vi) halogenated.
Further, 5-nitro-6-methyl-2-acetamido-pyrimidin-4-one can be sequentially (i) chlorinated, preferably with a reagent such as phosphoryl chloride; (ii) reacted with alkoxide to displace chloride and effect N-deacetylation; (iii) treated with a reagent capable of delivering a formyl equivalent, especially a dimethylaminomethylating agent, preferably Bredereck""s reagent; (iv) treated with a reducing agent to reduce the nitro group and cause cyclization; (v) N-protected on the pyrrole nitrogen; (vi) saponified to remove an N-formyl group; and in either order (vi) N-protected and (viii) halogenated.
Compounds of the formula (II) defined above may also be prepared by known methods, as described in WO 99/19338 and the references cited therein.
Suitable reagents for halogenation of a compound of formula (II) include chlorinating or brominating agents, and these include N-chloro- and bromoamides, chlorine and bromine, preferably N-chlorosuccinimide. Halogenation is conveniently carried out at ambient temperatures in an alkane as solvent, preferably hexane, more preferably pentane. Where the halogenation reagent is N-chlorosuccinimide, the succimide byproduct and any excess reagent can be removed by filtration. An excess of the halogenation reagent can be employed, though it is preferable to use close to equimolar quantity.
Suitable sterically hindered bases that can be used to form the imine by dehalogenation include alkali metal salts of bulky alcohols or amines, such as potassium tert-butoxide, lithium diisopropylamide or preferably lithium tetramethylpiperadide. An excess of base can be employed, though it is preferable to use close to an equimolar quantity. Preferably the amount of base used is determined experimentally as just sufficient to result in complete reaction of the compound of formula (XIX), and this can be judged by thin layer chromatography.
The imine formed by halogenation and dehydrohalogenation of a compound of formula (II) is more stable when kept at room temperature or below, but does not readily condense with the anion produced by abstraction of bromine or iodine from a compound of formula (XIX) at temperatures below xe2x88x9240xc2x0 C. The anion can be prepared at temperatures of xe2x88x9235 to xe2x88x9275xc2x0 C., but the temperature of the reaction medium should be in the range of xe2x88x9220 to +10xc2x0 C. to effect the condensation reaction. The anion is unstable at temperatures above +10xc2x0 C., and is preferably kept at temperatures below 0xc2x0 C., more preferably at or below xe2x88x9210xc2x0 C. The anion can be more stable in diethyl ether solution, and this is the preferred solvent. Compounds of formula (XIX) and the anions formed from them can have Limited solubility in diethyl ether, however, so that addition of a further solvent to assist with solubility is sometimes necessary. In this case the favoured solvent is anisole, so that the favoured reaction medium is a mixture of diethyl ether and anisole, the proportions being chosen to optimize solubility and stability of the reactants. An excess of either the anion or the imine can be employed, though it is preferable to use close to equimolar quantities of these reactants. As a small portion of the anion can be quenched by proton abstraction reactions or be subject to degradation reactions at the temperatures required to effect coupling, it is sometimes preferable to use a small excess of the anion, up to 2 equivalents, preferably up to 1.2 equivalents.
Examples of preferred reagents for performing the abstraction of the bromine or iodine atom from the compound of formula (XIX) are butyllithium or magnesium, although other suitable reagents will be apparent to those skilled in the art.
The above condensation reaction produces a compound of the formula (XX) as defined above.
In some instances, in order to facilitate purification, a derivative of formula (XX) wherein Zxe2x80x2 is a trialkylsilyloxy, alkyldiarylsilyloxy or optionally substituted triarylmethoxy group (such as trityloxy (ie unsubstituted triphenylmethoxy) or 4-monomethoxy or 4,4xe2x80x2-dimethoxytrityloxy) and R9 is a hydrogen atom can be further converted into a derivative of formula (XX) wherein Zxe2x80x2 is a hydroxy group and R9 is a hydrogen atom. For example, in the case wherein Zxe2x80x2 is a trialkylsilyloxy or alkyldiarylsilyloxy group, preferably a tert-butyldimethylsilyloxy group, this can be achieved by treatment with tetrabutylammonium fluoride in tetrahydrofuran followed by chromatography.
In some instances, in order to facilitate purification, a derivative of formula (XX) wherein R9 is a hydrogen atom can be further converted into a derivative of formula (XX) wherein R9 is an alkoxycarbonyl or aralkyloxycarbonyl group, preferably a tert-butoxycarbonyl group, for example by treatment with di-tert-butyl dicarbonate in methylene chloride followed by chromatography.
The compound of formula (XX) (either prepared directly from the condensation reaction or from subsequent conversion to another compound of formula (XX) as described immediately above) is then N- and O-deprotected by acid- or alkali-catalyzed hydrolysis or alcoholysis or catalytic hydrogenolysis as required for the O- and N-protecting groups in use, to produce a compound of the formula (I) as defined above.
Where R6 is a trialkylsilyl (preferably a tert-butyldimethylsilyl), alkyldiarylsilyl or 2-trimethylsilylethoxyethyl group, this group can be removed with a source of fluoride, such as tetrabutylammonium fluoride or hydrogen fluoride pyridine complex, in a solvent such as tetrahydrofuran.
Where Bxe2x80x2 is a benzyloxy group, and/or R6 is a benzyloxymethyl group, and/or R8 is a benzyl or p-methoxybenzyl group, and/or R9 is an aralkyloxycarbonyl (preferably a benzyloxycarbonyl) group, deprotection can be effected by hydrogenolysis over a metal catalyst. A suitable catalyst is palladium on charcoal, and suitable solvents are ethyl acetate, ethanol and methanol.
Where R6 is a benzyloxymethyl group it can be removed by treatment with a strong acid, such as concentrated hydrochloric acid, the excess acid being removed by evaporation, suitably under reduced pressure. Alternatively it can be removed by hydrogenolysis over a metal catalyst. A suitable catalyst is palladium on charcoal, and suitable solvents are ethyl acetate, ethanol and methanol. Intermediates in these process are compounds wherein R6 is a hydroxymethyl group. This group can resist further reaction under the above conditions but can readily be removed by alkali treatment. Suitable alkaline conditions are ammonia or an alkylamine (such as triethylamine) in water or alcohol solution at room temperature or up to 100xc2x0 C. The aforementioned hydrogenolysis can be conducted under alkaline conditions to effect full deprotection.
Where Bxe2x80x2 is a methoxy, tert-butoxy or benzyloxy group, and/or Zxe2x80x2 is a trialkylsilyloxy (preferably a tert-butyldimethylsilyloxy) or alkyldiarylsilyloxy group, and/or R6 is a trialkylsilyl (preferably a tert-butyldimethylsilyl), alkyldiarylsilyl, 2-trimethylsilylethoxymethyl or benzyloxymethyl group, and/or R9 is an alkoxycarbonyl or aralkyloxycarbonyl group, especially a tert-butoxycarbonyl group, deprotection can be effected by treatment with aqueous, alcoholic or concentrated acid. Suitable acids are hydrochloric or trifluoroacetic acids. The reaction can be conducted in the range 20-120xc2x0 C., preferably in concentrated aqueous hydrochloric acid under reflux.
The intermediate compounds of the formula (XX) are novel and constitute a further aspect of the invention.
The compounds of formula (XX) may be prepared by the methods described above. The particular reaction conditions suitable for the production of compounds of formula (XX) will depend upon the particular derivative concerned.
Examples of compounds of the formula (XX) of the invention include the following. The numbering of the table corresponds to the numbering in the Examples which will follow.
One example of the process of the present invention is a process of preparing the compound (1S)-1,4-dideoxy-1-C-(2,4-dihydroxypyrrolo[3,2-d]pyrimidin-7-yl)-1,4-imino-D-ribitol. In this method the compound of formula (II) is 5-O-tert-butyldimethylsilyl-1,4-dideoxy-1,4-imino-2,3-O-isopropylidene-D-ribitol, which is reacted with N-chlorosuccinimide and lithium tetramethylpiperidide to form an imine. The imine is condensed with the anion prepared by abstraction of the bromine atom from the compound 7-bromo-5-N-tert-butyldimethylsilyl-2,4-dibenzyloxypyrrolo[3,2-d]pyrimidine using butyl-lithium. The resulting protected product is then subjected to hydrogenolysis in ethanol over palladium in charcoal followed by acid-catalysed alcoholysis in methanol to perform the N- and O-deprotection and produce the compound (1S)-1,4-dideoxy-1-C-(2,4-dihydroxypyrrolo[3,2-d]pyrimidin-7-yl)-1,4-imino-D-ribitol as a salt.
Another example of the process of the present invention is a process of preparing the compound (1S)-1,4-dideoxy-1-C-(4-hydroxypyrrolo[3,2-d]pyrimidin-7-yl)-1,4-imino-D-ribitol (compound 3). In this process the compound of formula (II) is 5-O-tert-butyldimethylsilyl-1,4-dideoxy-1,4-imino-2,3-O-isopropylidine-D-ribitol, which is reacted with N-chlorosuccinimide and lithium tetramethylpiperidide to form an imine. The imine is condensed with the anion prepared by abstraction of the bromine atom from the compound 5-N-benzyloxymethyl-7-bromo-4-methoxypyrrolo[3,2-d]pyrimidine using butyllithium. The resulting protected product is then subjected to acid catalysed hydrolysis to perform the deprotection and produce the compound (1S)-1,4-dideoxy-1-C-(4-hydroxypyrrolo[3,2-d]pyrimidin-7-yl)-1,4-imino-D-ribitol.
In the above process, the compound 5-N-benzyloxymethyl-7-bromo-4-methoxypyrrolo[3,2-d]pyrimidine may be prepared by carrying out N,O-protection of the compound 3H,5H-pyrrolo[3,2-d]pyrimidin-4-one and brominating the protected compound at C7.
In further aspects, the present invention provides new processes for preparing other intermediate compounds (besides those of formula (XX)), useful in the process defined above of preparing compounds of formula (I). It also provides certain novel intermediate compounds useful in this process. These aspects of the present invention will now be described in more detail.
In particular, the present invention in certain aspects relates to new processes for preparation of the compounds 3H,5H-pyrrolo[3,2-d]pyrimidin-4-one and 2-amino-3H,5H-pyrrolo[3,2-d]pyrimidin-4-one (1 and 2 shown below). 
These two compounds are important 9-deaza-isosteres of hypoxanthine and guanine. They are also known inhibitors of purine nucleoside processing enzymes. We have also found that these compounds are useful intermediates which can be used to prepare compounds of the formula (XIX) as defined above, which in turn can be used to prepare the compounds 3 and 4 defined above, respectively, using the convergent synthetic process of the invention for preparing compounds of the formula (I) as defined above. The compounds 3 and 4 are compounds of the general formula (I) which are extremely potent inhibitors of purine nucleoside phosphorylase.
There are relatively few routes to the pyrrolo[3,2-d]pyrimidine ring system.2 Most start from pyrimidines with appropriate functional groups at the 5 and 6 positions. The first syntheses of 1 and 2 were lengthy and proceeded in poor overall yield.3 Other syntheses of 1 and 2 have since been reported4-7 but none were as simple as we desired.
We have invented new facile syntheses of 1 and 2 that allow ready access to these compounds on a multi-gram scale.
A previous synthesist of 2 (shown in Reaction Scheme 1) started from 2-amino-6-methyl-5-nitropyrimidin-4-one 5 (readily available8 by nitration of the commercially available 2-amino-6-methylpyrimidin-4-one) which was protected as 6. Chromatography was required at this point to separate the N- and O-pivaloyloxymethyl isomers. Formylation of the 6-methyl group was then accomplished and reduction of 7 with sodium dithionite afforded the pyrrolo[3,2-d]pyrimidine ring system in 8 which was deprotected to give 2. Attempts to formylate the 6-methyl group of 5 directly led only to the N-methyl derivative 9. We have reinvestigated this reaction and now report conditions under which 5 is directly converted into 10 without any apparent formation of the N-methyl compound 9. In particular, 5 may be converted to 10 by reaction with a reagent capable of effecting dialkylaminomethylation. It is preferred that the reagent used is DMF and DMF dimethylacetal. However, those persons skilled in the art will appreciate that alternative reagents, such as Bredereck""s reagent, may be used. Reduction of this compound 10 by boiling in aqueous sodium dithionite solution then afforded 2 directly in good yield without recourse to chromatography. Formation of the N-methyl compound is apparently dependent on the concentration of N,N-dimethylformamide used. Lesser amounts will lead to some 9.
It will also be appreciated that the reduction of compound 10 to afford 2 may be effected using any suitable reagent capable of reducing the nitro group an 10. Although a preferred reagent is aqueous sodium dithionite solution, other means of effecting the reduction, such as by catalytic hydrogenation, are within he scope of the invention. 
Extension of this approach to the synthesis of 1 would require 11 (shown in Reaction Scheme 2) as starting material but it is difficult to obtain. Another approach that has been used to synthesize 7-substituted pyrrolo[3,2-d]pyrimidines such as 9-deazainosine utilized a 2-ethoxycarbonyl-3-aminopyrrole 12 which cyclized readily on treatment with formamidine acetate to give 13.9 The same treatment applied to pyrrole 14 should produce 1 but 14 has not yet been reported. A useful synthesis of 4-substituted pyrrole 15 has been described7 whereby the masked aldehyde ethyl (ethoxymethylene)cyanoacetate 16 was treated with diethyl aminomalonate 17 under basic conditions. However, when 3-ethoxyacrylonitrile was treated with 17 under the same conditions there was no sign of pyrrole 14 being formed. It is known10 that isoxazole 18 will react under basic conditions to produce the unstable 3-oxopropionitrile 19 as a transient intermediate. We have found that treatment of 19 without isolation with 17 gave an intermediate presumed to be 20 as a stereoisomeric mixture. Further treatment of this with sodium ethoxide in ethanol produced pyrrole 14 in good overall yield. When 14 was allowed to react with formamidine acetate in refluxing ethanol the pyrrolo[3,2-d]pyrimidine 1 was formed in high yield. This new synthesis of 1 from isoxazole is facile and can be effected without recourse to chromatography. The ready availability of 14 in this way would also allow access to 2 by known methods7,11 but the above route is superior in our experience.
The process according to this aspect of the invention also extends to the synthesis of other compounds of the formula (A) defined above in which Y is an unsubstituted or substituted alkyl or arylalkyl group having 1 to 8 carbon atoms. These compounds are novel and constitute a further aspect of the invention. Examples of suitable substituents for Y are F, Cl or OMe.
In general terms, the compounds of the formula (A) can be prepared by reacting isoxazole 18 with alkoxide ions in the presence of an alcohol, quenching the reaction with an acid, reacting the resulting reaction mixture with a dialkyl aminomalonate; isolating an organic phase of the resulting reaction mixture and reducing it to a residue, and reacting the residue with a base in a protic or aprotic solvent to produce a compound of the formula (A). It is generally preferred that this final step involves reaction with alkoxide ions in excess alcohol. When the compound 14 itself is to be prepared, the alcohol used in excess in this step is ethanol and the alkoxide ions are ethoxide ions (as shown in Reaction Scheme 2). Compounds of the formula (A) in which Y is other than ethyl can be prepared by selecting an appropriate alcohol corresponding to the desired group Y.
The compound 1 may be prepared from the compound of formula (A) thus prepared by reaction with a reagent capable of delivering a formyl equivalent. Conveniently, this reagent may be formamidine acetate although other suitable reagents will be apparent to those skilled in the art.
In conclusion, we have invented a synthesis of the previously unknown compounds of formula (A) from which the pyrrolopyrimidine 1 is now readily available. We have also invented a shorter synthetic approach to 2 so that it is much more readily available. 
The invention thus in a further embodiment also relates to a method of preparing the compound 3 or 4 defined above via the intermediate compound 3H,5H-pyrrolo[3,2-d]pyrimidin-4-one (1) or 2-amino-3H,5H-pyrrolo[3,2-d]pyrimidin-4-one (2). The compound 3 or 4 can be prepared by first carrying out N,O-protection of the compound 1 or 2, brominating the protected compound, followed by lithiation of the resulting brominated compound, addition of the resulting lithiated species to a protected imine, specifically a 2,3,5-tri-O-protected 1,N-dehydro-1,4-dideoxy-1,4-imino-D-ribitol, and N,O-deprotection of the resulting species to form the compound 3 or 4. Typically, the protecting groups used to protect the imine may be selected from trialkylsilyl, arylalkyl and isopropylidene groups. This synthetic route with preferred reagents to prepare the compound 3 is shown in Reaction Scheme 3. Other suitable reagents and reaction conditions for each stage of the synthesis will be apparent to those persons skilled in the art. 
The invention will now be further described with reference to the following non-limiting examples.